<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .c1,#outer,.c2{
            color: red;
        }
    </style>
</head>
<body>
    <div class="c1">
        c1...
    </div>

    <div id="outer">
        outer
    </div>

    <p class="c2">
        p-c2
    </p>
</body>
</html>